Process for thick film circuit patterning

ABSTRACT

A process for patterning thick film electrically functional patterns using a photosensitive polymer layer. A tacky photosensitive layer is applied onto a substrate surface. The photosensitive layer is imaged with a pattern using actinic radiation, the exposed areas of the photosensitive layer become hardened and non-tacky. A subsequent application of a thick film composition sheet will cause the thick film to adhere to the remaining tacky areas. Upon peeling the sheet, a thick film print pattern will be produced. This step is followed by a processing profile prescribed by the thick film composition used which results in a pattern having electrically functional properties. The invention also extends to a process wherein a thick film composition is recovered from a used sheet.

FIELD OF THE INVENTION

[0001] The invention relates to forming an electrically functionalpattern on a substrate. More specifically, the invention relates to aprocess for using a photosensitive element in combination with a sheethaving a thick film composition applied to a support. The invention alsorelates to a process for recovering the thick film composition from aused sheet.

BACKGROUND OF INVENTION

[0002] Established patterning methods used in the thick film industryinclude printing (screen or other), spraying, brushing, spin coating,dipping, laminating, photo-patterning and etching. Screen-printing is byfar the most commonly used method for patterning thick film circuits. Acertain amount of a thick film composition is placed at the edge of aprinting screen wherein a squeegee moves and presses the thick filmcomposition across the screen. A circuit pattern similar to the screenopenings is printed on the substrate. The printed substrate is thendried which causes most of the solvent in the thick film composition toevaporate, and then fired which causes burnout of any organic vehicle.Although screen print pattern resolution can be improved by optimizingscreen wire-mesh dimensions, emulsion type, thickness, and screenprinting variables; there is an accepted opinion in the industry thatscreen printed fine line and space resolution is not adequate forcertain applications, since screen printed line resolutions are about125 microns or larger.

[0003] Photo-patterning and etching approaches are alternativepatterning technologies that offer uniform finer lines and spaceresolution when compared to screen-printing. A photo-patterning method,such as DuPont's FODEL® printing system, utilizes a photosensitiveorganic medium as found in patents U.S. Pat. No. 4,912,019; U.S. Pat.No. 4,925,771; U.S. Pat. No. 5,032,490, whereby the substrate is firstcompletely covered (printed, sprayed, coated or laminated) with thephotosensitive thick film composition and dried. An image of the circuitpattern is exposed by actinic radiation through a photomask bearing acircuit pattern, the exposed substrate is then developed. The unexposedportion of the circuit pattern is washed away leaving a patternedsubstrate that is fired to remove all remaining organic materials andsinter inorganic materials. Such a photo-patterning method demonstratesthick film line resolution of about 30 microns or larger depending onthe substrate smoothness, inorganic particle size distribution, exposureand development variables. The narrow window of particle sizedistribution, circuit line conductivity, thickness and circuit linethickness uniformity are limitations of this thick film photo-patterningmethod.

[0004] Another system of patterning is an etching method, a thick filmcomposition is printed, dried and fired on a substrate surface. Aphotoresist is then applied on top of the fired thick film surface thatis sequentially exposed and developed. The substrate is then dipped orsprayed with a chemical that etches the fired metallization in the openareas of the photoresist. After washing and resist removal, a desiredcircuit pattern is produced on the substrate. Line capabilities of etchpatterning is about 10 microns or larger depending on the substratesmoothness, particle size distribution of the inorganic powders, typeand activity of the etching solution and photoresist adhesion. The useof hazardous chemical etching solutions and remnant inorganic binderfound on top of the substrate surface are some limitations of the thickfilm etch patterning approach for display and other applications.

[0005] Yet, another system is described in U.S. Pat. No. 5,110,384, U.S.Pat. No. 5,167,989 and U.S. Pat. No. 5,296,259. These patents describe adry powder method of applying conductive or phosphor particles in apattern to a substrate. Precleaned ceramic substrates are coated with athin layer of a phototackifiable material, exposed imagewise with UVlight, toned with metal powder toner material and fired according toconventional thick film firing profiles.

[0006] The graphic arts industry widely uses photosensitive elements tocreate color patterns of an image. The elements usually comprise asupport, a photosensitive layer and a cover sheet. The photosensitivelayer of the element either looses tackiness or becomes tacky uponexposure to actinic radiation. The exposure of the photosensitive layeris used to create a latent image which is then formed by applying tonerparticles by way of a powder or a toner sheet to the imagewise exposedelement. The toner particles stick to the tacky areas so that an imageis created by the toner particles adhered to the tacky areas. U.S. Pat.No. 3,649,268 describes a reproduction process using positive-workingphotosensitive elements. Negative-working photosensitive elements arealso known which require a dual exposure to provide a negative imagewith tacky and non-tacky areas. Negative-working elements and theirprocess of use are disclosed in U.S. Pat. Nos. 4,174,216 and 4,247,619.

[0007] The present invention combines the concept of forming an imagedpattern on a substrate using a photosensitive element used in thegraphics art industry and a thick film composition used in establishedmethods of pattern formation in the electronics industry. The inventiondoes not include the step of developing a photosensitive element that isneeded to form color images in the graphics art industry. It has beenfound that the invention produces high quality circuit patterns withsubstantially no shrinkage, edge curl or undercut at a low cost in anenvironmentally compatible manner.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to a process for forming apattern having electrically functional properties on a substratecomprising the steps of: (a) providing a photosensitive layer having atacky surface disposed on a substrate; (b) image-wise exposing thephotosensitive tacky surface to form an imaged layer having tacky andnon-tacky areas; (c) applying a sheet comprising at least one layer of athick film composition disposed on a support to the imaged layer whereinthe imaged layer is in contact with the thick film composition of thesheet; (d) removing the support wherein the thick film compositionremains on the support in the non-tacky areas of the imaged layer andthe thick film composition substantially adheres to the tacky areas ofthe imaged layer forming a patterned article; and (e) firing the thickfilm composition of the patterned article.

[0009] The above process also may be practiced without exposure of atacky surface resulting in full coverage of the tacky surface by thethick film composition.

[0010] The present invention is further directed to a process forforming a pattern having electrically functional properties on asubstrate comprising the steps of: (a) providing a photosensitive layerhaving a tacky surface disposed on a substrate; (b) image-wise exposingthe photosensitive tacky surface to form an imaged layer having tackyand non-tacky areas; (c) applying the exposed photosensitive layer to asheet comprising at least one layer of a thick film composition disposedon a support wherein the imaged layer is in contact with the thick filmcomposition of the sheet; (d) removing the imaged layer wherein thethick film composition substantially adheres to the tacky areas of theimaged layer and remains on the support in the non-tacky areas of theimaged layer forming a patterned article; (e) curing the thick filmcomposition of the patterned article.

[0011] The present invention is still further directed to a process forthe recovery of a thick film composition from a support comprising asupport having a dried thick film composition in the form of a reversecircuit pattern comprising the sequential steps of: (a) passing thedried thick film composition on a first support through a solvent bathto form a solution of the thick film composition; (b) adjusting theviscosity of the solution to form a castable solution; and (c) applyingthe castable solution on a second support.

[0012] The present invention is still further directed to a method forusing a photosensitive layer to form a pattern having electricallyfunctional properties with a thick film composition, comprising thesteps of: (a) forming tacky and non-tacky areas on the surface of aphotosensitive layer by image-wise exposing the photosensitive layer toactinic radiation to form an image-wise exposed surface; (b) contactingthe image-wise exposed surface with a thick film composition whichsubstantially adheres to the tacky areas but does not adhere to thenontacky areas to form a pattern of thick film composition on theimage-wise exposed surface; and (c) firing the thick film composition onthe image-wise exposed surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is an illustrative diagram depicting an embodiment of theprocess of the present invention.

[0014]FIG. 2 is an illustrative diagram depicting an embodiment of theprocess of the present invention.

[0015]FIG. 3 is an illustrative diagram depicting an embodiment of therecovery process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Generally, a thick film composition comprises a functional phasethat imparts appropriate electrically functional properties, such as,conductive, resistive and dielectric properties. The functional phasecomprises electrically functional powders dispersed in an organic mediumthat acts as a carrier for the functional phase. The functional phasedetermines the electrical properties and influences mechanicalproperties of a dried thick film. There are two main types of thick filmcompositions that may be utilized in this invention. Both areconventional products sold in the electronics industry. First, thickfilm compositions wherein the organics of the compositions duringprocessing is burned or fired out are referred to as “firable thick filmcompositions”. They typically comprise conductive, resistive ordielectric powders and inorganic binder dispersed in organic medium.Prior to firing, a processing requirement may include an optional heattreatment such as: drying, curing, reflow, soldering and others known tothose skilled in the art of thick film technology. Second, thick filmcompositions that typically comprise conductive, resistive or dielectricpowders and are dispersed in organic medium wherein the compositionsduring processing are cured and the organics remains are referred to as“polymer thick film compositions”. Fireable thick film compositions andpolymer thick film compositions are generally referred to as “thick filmcompositions”. “Organics” comprise polymer or resin components of athick film composition.

[0017] In conductor applications the functional phase is comprised ofelectrically functional conductor powder(s). The electrically functionalpowders in a given thick film composition may comprise a single type ofpowder, mixtures of powders, alloys or compounds of several elements.Examples of such powders include: gold, silver, copper, nickel,aluminum, platinum, palladium, molybdenum, tungsten, tantalum, tin,indium, lanthanum, gadolinium, boron, ruthenium, cobalt, titanium,yttrium, europium, gallium, sulfur, zinc, silicon, magnesium, barium,cerium, strontium, lead, antimony, conductive carbon, and combinationsthereof and others common in the art of thick film compositions.

[0018] In resistor compositions, the functional phase is generally aconductive oxide. Examples of the functional phase in resistorcompositions are Pd/Ag and RuO₂. Other examples include rutheniumpyrochlore oxide which is a multi-component compound of RU⁺⁴, IR⁺⁴ or amixture of these (M″), said compound being expressed by the followinggeneral formula:

(M_(x)Bi_(2-x)) (M′_(y)M″_(2-y))0_(7-z)

[0019] wherein

[0020] M is selected from the group consisting of yttrium, thallium,indium, cadmium, lead, copper and rare earth metals,

[0021] M′ is selected from the group consisting of platinum, titanium,chromium, rhodium and antimony,

[0022] M″ is ruthenium, iridium or a mixture thereof,

[0023] x denotes 0 to 2 with a proviso that x≦1 for monovalent copper,

[0024] y denotes 0 to 0.5 with the proviso that when M′ is rhodium ortwo or more of platinum, titanium, chromium, rhodium and antimony, ystands for 0 to 1, and

[0025] z denotes 0 to 1 with a proviso that when M is divalent lead orcadmium, z is at least equal to about x/2.

[0026] These ruthenium pyrochlore oxides are described in detail in thespecification of U.S. Pat. No. 3,583,931. The preferred rutheniumpyrochlore oxides are bismuth ruthenate (Bi₂Ru₂O₇) and lead ruthenate(Pb₂Ru₂O₆).

[0027] In dielectric compositions, the functional phase is generally aglass or ceramic. Dielectric thick film compositions are nonconductingcompositions or insulator compositions that separate electrical chargesand may result in the storage of an electrical charge. Therefore, thethick film dielectric compositions typically contain ceramic powders,oxide and non-oxide frits, crystallization initiator or inhibitor,surfactants, colorants, organic mediums, and other components common inthe art of such thick film dielectric compositions. Examples of ceramicsolids include: alumina, titanates, zirconates and stannates, BaTiO₃,CaTiO₃, SrTiO₃, PbTiO₃, CaZrO₃, BaZrO₃, CaSnO₃, BaSnO₃ and Al2O₃, glassand glass-ceramic. It is also applicable to precursors of suchmaterials, i.e., solid materials which upon firing are converted todielectric solids, and to mixtures thereof.

[0028] The powders described hereinabove are finely dispersed in anorganic medium and are optionally accompanied by, inorganic binders,metal oxides, ceramics, and fillers, such as other powders or solids.The function of an inorganic binder in a thick film composition isbinding the particles to one another and to the substrate after firing.Examples of inorganic binders include glass binders (frits), metaloxides and ceramics. Glass binders useful in the thick film compositionare conventional in the art. Some examples include borosilicates andaluminosilicates glasses. Examples further include combinations ofoxides, such as: B₂O₃, SiO₂, Al₂O₃, CdO, CaO, BaO, ZnO, SiO₂, Na₂O, PbO,and ZrO which may be used independently or in combination to form glassbinders. Typical metal oxides useful in thick film compositions areconventional in the art and can be, for example, ZnO, MgO, CoO, NiO,FeO, MnO and mixtures thereof.

[0029] The functional phase and any other powders are typically mixedwith an organic medium by mechanical mixing to form a pastelikecomposition having suitable consistency and rheology for printing. Awide variety of inert liquids can be used as organic medium. The organicmedium must be one in which the solids are dispersible with an adequatedegree of stability. The rheological properties of the medium must besuch that they lend good application properties to the composition. Suchproperties include: dispersion of solids with an adequate degree ofstability, good application of composition, appropriate viscosity,thixotropic, appropriate wettability of the substrate and the solids, agood drying rate, good firing properties, and a dried film strengthsufficient to withstand rough handling. The organic medium isconventional in the art and is typically a solution of the polymer insolvent(s). The most frequently used resin for this purpose is ethylcellulose. Other examples of resins include ethylhydroxyethyl cellulose,wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and monobutyl ether of ethyleneglycol monoacetate can also be used. The most widely used solvents foundin thick film compositions are ethyl acetate and terpenes such as alpha-or beta-terpineol or mixtures thereof with other solvents such askerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,hexylene glycol and high boiling alcohols and alcohol esters. Variouscombinations of these and other solvents are formulated to obtain theviscosity and volatility requirements desired.

[0030] In addition, the thick film composition can also include othermetal particles and inorganic binder particles to enhance variousproperties of the composition, such as adhesion, sintering, processing,brazeability, solderability, reliability, etc. during processing. Oxalicacid catalyzed alkyl t-butyl/amyl phenolic resin is an example of anadhesion promoter used to increase adhesion of the thick filmcomposition to a support of a transfer sheet which is further describedhereinbelow.

[0031] In a fireable thick film composition, when firing in the 300 to1000° C. temperature range, adhesion of the thick film composition tothe substrate is generally achieved by the melted glass frit(s) wettingthe substrate. The inorganic binder (glass frits, metal oxides and otherceramics) portion of the thick film composition is the focus of adhesionto the substrate. For example, in a traditional thick film conductorcomposition firing, the sintered metal powders are wetted or interlockedby the inorganic binder, at the same time, the inorganic binder wets orinterlocks with the substrate, thus, producing adhesion between thesintered metal powders and the substrate. Hence, for thick filmfunctionality, it is important that the patterning technology deposits awell dispersed thick film composition with all the necessary ingredientswithin prescribed quantities. For firing temperatures above 1000° C., inaddition to inorganic binder wetting/interlocking adhesion mechanisms,other interactions and compound formation could contribute to adhesionmechanisms.

[0032] Polymeric thick film compositions are mainly made up ofconductive, resistive or dielectric powders, such as those discussedhereinabove, dispersed in an organic medium containing polymer ornatural and synthetic resin and solvent, typically volatile solvent anda polymer. They typically do not include glass frit since they are curedand not fired. Some examples of typical polymers employed in polymericthick film compositions are polyesters, acrylics, vinyl chlorides, vinylacetates, urethanes, polyurethanes, epoxies, phenolic resin systems, ormixtures thereof. The organic medium is preferably formulated to giveappropriate wettability of the particles and the substrate, good dryingrate, dried film strength sufficient to withstand rough handling.Satisfactory appearance of the dried composition is also important.

[0033] Solvents suitable must dissolve the polymer. Some examples ofsolvents are listed: propylene glycol monomethyl ether acetate, methylpropanol acetate, 1-methoxy-2 propanol acetate, methyl cellosolveacetate, butyl propionate, primary amyl acetate, hexyl acetate,cellosolve acetate, pentyl propionate, diethylene oxalate, dimethylsuccinate, dimethyl glutarate, dimethyl adipate, methyl isoamyl ketone,methyl n-amyl ketone, cyclohexanone, diacetone alcohol, diisobutylketone, n-methyl pyrolidone, butyrolactone, isophorone, methyln-isopropyl ketone. Various combinations of these and other solvents areformulated to obtain the desired viscosity and volatility requirementsfor the process that the polymer thick film composition is to beemployed.

[0034] The organic medium is required to impart the necessary adhesionto the desired substrate, and it, also, provides the composition withthe required surface hardness, resistance to environment changes andflexibility. Additives as known to those skilled in the art may beemployed in the organic medium to fine-tune the viscosity for printing.

[0035] After applying a polymer thick film composition on a basematerial, the composition is typically dried by heating at temperaturesof up to about 150° C. which cause the volatile solvents to be drivenoff or dried. After drying, depending on the application, thecomposition will undergo a curing process wherein the polymer will bindthe powder to form a circuit pattern or other desired result. In orderto obtain the desired end properties, one skilled in the art knows it isimportant that the thick film composition contains an optimized amountof each of the desired ingredients to meet the end result. For example,a thick film silver composition for varistor termination applications,may contain 70+ or −2 percent of a specific silver powder, 2+ or −0.04percent of a mixture of frits that are compatible with the type ofvaristor ceramic substrate used, 0.5+ or −0.01 percent of metal oxideadhesion promoter, sintering promoter/inhibitor and the balance beingorganic medium consisting of polymer(s), solvent(s), surfactant(s),dispersant(s) and other materials commonly used in the art of thick filmcompositions. The optimized amount of each ingredient is important toachieve the desired thick film conductor, resistor, dielectric oremitter properties. The properties needed may include coverage, density,uniform thickness and circuit pattern dimensions, electrical propertiessuch as: resistivity, current-voltage-temperature characteristics,microwave, radio-high frequency characteristics, capacitance,inductance, etc.; interconnection characteristic properties, such as:solder or braze wetting, compression and wire bonding, adhesive joining,and junction characteristics; optical properties, such as: fluorescence;and other initial and aged/stress testing properties that may berequired.

PROCESS DESCRIPTION AND MATERIAL SYSTEMS

[0036] The process of the present invention comprises a photosensitivepolymer layer that is applied onto a substrate surface. In a patterningprocess, a pattern is imaged onto a tacky photosensitive polymer layerusing actinic radiation; the exposed areas of the polymer layer undergoa chemical change that renders the areas non-tacky. A subsequentapplication of a thick film transfer sheet, preferably by lamination,will cause a thick film composition which has electrically functionalproperties to adhere only at the tacky patterned areas. Upon peeling offthe transfer sheet, a thick film print of the pattern will be producedon top of the tacky areas of the imaged photosensitive layer. Typicalprocessing conditions as prescribed by the thick film composition usedon the transfer sheet will then be followed.

[0037] The new thick film patterning approach comprises the followingmaterials and process steps:

[0038] A sheet, referred to as a transfer sheet for illustrationpurposes, is depicted by FIG. 1(a). It comprises at least one layer of adried-strippable thick film composition (101), preferably a fireablethick film composition, with powders, inorganic binders and organicmediums as found in the thick film compositions as describedhereinabove, deposited on a support (102).

[0039] The thick film composition is deposited, for example, by casting,printing or spraying on a strippable support and then dried. Duringdrying the volatile organic solvents are evaporated. The support is adelivery vehicle for applying the dried thick film composition to animaged photosensitive layer. The dried-strippable thick film compositionlayer should have sufficient adhesion to the support to remain affixedto the support throughout the required process steps, but at the sametime, the adhesive strength of the dried-strippable layer should becarefully balanced with the adhesive strength of the strippable supportso the thick film composition could be deposited on an imagedphotosensitive layer to carryout the steps in the process of theinvention.

[0040] The strippable support may comprise almost any material that hasreasonable flexibility and integrity. A single layer or multiple layersof a thick film composition may be applied to the support. The supportis generally smooth and flat and dimensionally stable. A polyester orpolyolefin film e.g. polyethylene polypropylene are examples of suitablesupports. Examples of suitable materials that can be used as a supportinclude MYLAR® polyester (polyethylene terepthalate) film available fromE. I. du Pont de Nemours and Company and TRESPAPHAN® film available fromHoechst, Winston-Salem, N.C. The support typically has a thickness of 10to 250 microns. The support may be in sheet form, which may beproportional to the size of the pattern that needs to be created or thesupport may be in a continuous roll. The roll will allow for continuousmass production. Optionally, a flexible cover sheet may be present onthe outmost layer of the dried thick film composition layer. The coversheet protects the underlaying areas and is easily removable.

[0041] In another embodiment of a transfer sheet, multiple layers ofthick film compositions may be deposited on a support resulting in adual-layer transfer sheet. A first layer of the multiple layers mayinclude a silver-containing thick film composition layer cast and driedonto a support. A second layer may include a black thick filmcomposition contrast layer cast and dried on top of the silver thickfilm layer resulting in two layers of thick film composition on onesupport.

[0042] In another embodiment of a transfer sheet, the components of thethick film composition could be separated. For example, a precious metalwith organic medium could be cast onto one support, and inorganicpowders with organic medium could be cast onto another support. Theprocess of the invention would then be accomplished in two steps.

[0043] The process employs a photosensitive layer having a tackysurface. The photosensitive layer comprises an optional strippablesupport or base layer, a photosensitive tacky layer and a strippablecover sheet, wherein the strippable support has greater adhesion to thephotosensitive tacky layer than the strippable cover sheet. Actinicradiation impinges on the photosensitive layer containing at least onephotoactive component to induce a physical or chemical change in thatmaterial. In the photosensitive compositions which are useful in thepresent invention, exposure to actinic radiation causes a change in thetackiness of the layer. This element would be a positive working elementas known in the art of photolithography. Examples would be CROMALIN®photosensitive products sold by E. I. du Pont de Nemours and Company,Wilmington, Del. Descriptions of positive working photosensitiveelements are disclosed in U.S. Pat. Nos. 3,649,268; 4,734,356 (positiveworking photosensitive elements including a support layer, aphotosensitive layer having a binder component, an ethylenicallyunsaturated monomer component and a photopolymerizable initiator, andoptionally a cover sheet); 4,849,322 (a multilayer element comprising acover sheet, photo-adherent layer and tonable contiguous layer);4,892,802; 4,948,704; 4,604,340 and 4,698,293.

[0044] In the case where the photosensitive compositions become lesstacky to non-tacky (hereinafter referred to as “non-tacky”) whenimage-wise exposed to actinic radiation, the composition is referred toas “photohardenable”. Photohardenable systems are well known andpreferred in the present invention and generally include aphotoinitiator or photoinitiator system (hereinafter referred tocollectively as “photoinitiator system”), and at least one compoundwhich reacts with the species generated by exposure of thephotoinitiator to actinic radiation, causing a decrease in tackiness, anethylenically unsaturated compound, and a binder. In this context, thephotoinitiator system, when exposed to actinic radiation, acts as asource of free radicals needed to initiate polymerization and/orcrosslinking of the ethylenically unsaturated compound. Although notlimited to photohardenable systems, the photosensitive layer of theelement of the invention will be further described in terms of suchsystems.

[0045] The photoinitiator system has one or more compounds that directlyfurnish free radicals when activated by actinic radiation. The systemalso may contain a sensitizer that is activated by the actinicradiation, causing the compound to furnish the free radicals. Usefulphotoinitiator systems can also contain a sensitizer that extendsspectral response into the near ultraviolet, visible, and near infraredspectral regions.

[0046] Photoinitiator systems are well known and discussions of suchsystems can be found in, for example, “Photoreactive Polymers: TheScience and Technology of Resists” by A. Reiser, John Wiley & Sons, NewYork, 1989, and “Radiation Curing: Science and Technology” edited by S.P. Pappas, Plenum Press, New York, 1992.

[0047] Preferred photoinitiator systems are free radical generatingaddition polymerization initiators activatable by actinic light andthermally inactive at and below 100° C. These include the substituted orunsubstituted polynuclear quinones such as 9,10-anthroquinone; vicinalketaldonyl alcohols, such as benzoin; α-hydrocarbon-substituted aromaticacyloins, including α-methylbenzoin; Michler's ketone, benzophenone,hexaarylbiimidazoles in association with hydrogen donors. Particularlypreferred photoinitiators include hexaarylbiimidazoles with hydrogendonors; Michler's ketone and ethyl Michler's ketone, particularly inassociation with benzophenone; and acetophenone derivatives.

[0048] The ethylenically-unsaturated compound is one which is capable ofundergoing free-radical initiated polymerization and/or crosslinking.Such compounds are generally known as monomers or oligomers, althoughpolymers having reactive pendant groups can also be used. Such compoundsare well known in the art and have been disclosed in, for example,“Light-Sensitive Systems: Chemistry and Application of Nonsilver HalidePhotographic Processes” by J. Kosar (John Wiley & Sons, Inc., 1965);“Imaging Processes and Materials—Neblette's Eighth Edition” edited by J.Sturge, V. Walworth and A. Shepp (Van Nostrand Reinhold, 1989); and“Photoreactive Polymer—The Science and Technology of Resists” by A.Reiser (John Wiley & Sons, 1989). Typical monomers are: unsaturatedesters of alcohols, preferably esters of polyols with acrylic ormethacrylic acid, such as t-butyl acrylate, cyclohexyl acrylate,hydroxy-C1-C10-alkyl acrylate, butanediol diacrylate, hexamethyleneglycol diacrylate, trimethylolpropane triacrylate, polyoxyethylatedtrimethylopropane triacrylate, diethylene glycol diacrylate, glyceroltriacrylate, ethylene glycol dimethacrylate, pentaaerythritol tri- andtetraacrylate and methacrylate; acryloxy- and methacryloxy-alkyl ethersof bisphenol A, such as di-(3-acryloxy-2-hydroxypropyl) ether ofbisphenol A and di-(3-acryloxy-2-hydroxypropyl) ether oftetrabromo-bisphenol A; unsaturated amides, such as 1,6-hexamethylenebisacrylamide; vinyl esters, such as divinyl succinate, divinylphthalate, and divinyl benzene-1,3-disulfonate; styrene and derivativesthereof; and N-vinyl compounds, such as N-vinyl carbazole.

[0049] The binder is a film forming material which may contain reactivegroups. Suitable binders that can be used alone or in combination arewell known in the art. These include polyacrylate and alpha-alkylacrylate esters; polyvinyl esters; ethylene vinyl acetate copolymers;polystyrene polymers and copolymers; vinylidene chloride copolymers;polyvinyl chloride and copolymers; synthetic rubbers; high molecularweight polyethylene oxides of polyglycols; epoxides; copolyesters;polyamides; polycarbonates; polyvinyl acetals; polyformaldehydes.Recently there has been more and more interest in binders which areaqueous processable. For aqueous processability, the binders should bedevelopable by aqueous alkaline solution. By “developable” is meant thatthe binders are soluble, swellable or dispersible in the developersolution. Preferably, the binder is soluble in the developer solution.Particularly preferred as binders are acidic, polymeric, organiccompounds. Single or multiple binder compounds can be used. One class ofbinders which is useful in the process of the invention is vinyladdition polymers containing free carboxylic acid groups. These areprepared from 30-94 mole percent of one or more alkyl acrylates and 70-6mole percent of one or more alpha-beta ethylenically unsaturatedcarboxylic acids; more preferably from 61-94 mole percent of two alkylacrylates and 39-6 mole percent of an alpha-beta ethylenicallyunsaturated carboxylic acid. Suitable alkyl acrylates for use inpreparing these polymeric binders include methyl acrylate, ethylacrylate, propyl acrylate, butyl acrylate, and the methacrylate analogs.Suitable alpha-beta ethylenically unsaturated carboxylic acids includeacrylic acid, methacrylic acid, crotonic acid, maleic acid or maleicanhydride, and the like. Binders of this type, including theirpreparation, are described in German Application OS 2,320,849, publishedNov. 8, 1973. Styrene can be substituted for one of the alkyl acrylateor methacrylate components. Also suitable are copolymers of styrene andsubstituted styrenes with an unsaturated carboxyl-containing monomer, asdescribed in detail in British Patent 1,361,298.

[0050] Other components conventionally added to photohardenablecompositions can be present to modify the physical properties of thelayer. Such components include: plasticizers, thermal stabilizers,optical brighteners, ultraviolet radiation absorbing material, colorformers, adhesion modifiers, coating aids, and release agents. Inaddition, depending on the application, other inert additives can beemployed such as dyes, pigments and fillers. These additives aregenerally present in minor amounts so as not to interfere with theexposure of the compositions.

[0051] Typical compositions for the photohardenable layer are, byweight, based on the total weight of the photohardenable layer:photoinitiator system, 0.1 to 10%, preferably 1 to 7%; ethylenicallyunsaturated compound(s), 5 to 60%, preferably 15 to 50%; binder(s), 25to 90%, preferably 45 to 75%; all other components, 0 to 5%, preferably0 to 4%. The thickness of the layer varies according to the end use.Generally, the thickness is in the range of 0.7 to 125 microns.

[0052] It is possible to have more than one photosensitive layer in thephotosensitive film. The layers can have the same or differentcompositions. It is also possible to have non-photosensitive layers, toadjust adhesion or other properties. The overall thickness of all thelayers, excluding the support and coversheet, should be in the samerange as that listed above for the single photosensitive layer, althoughgreater thicknesses can be used.

[0053]FIG. 1(b) illustrates an assembly wherein a removable base layerwas removed from a photohardenable layer (104) that has a tacky surfaceand an optional cover layer (103) such as MYLAR® film, followed bylaminating the photohardenable layer onto a substrate (105). Substratesthat may be used in the assembly could be rigid or flexible, andpermanent or temporary, and are known by those skilled in the art ofcircuit assembly. Some examples of substrates include: glass panels (forexample, a soda lime glass), glass-ceramic, low-temperature co-firedceramics, alumina, aluminum oxide, and coated substrates, such asporcelainized steel, glazed ceramic substrates, and insulated metalsubstrates which are insulated with ceramic, glass or polymer. Thesubstrates could be in their fired or green state. The photohardenablelayer is sandwiched between the substrate and the cover layer. The coverlayer is transparent for actinic radiation penetration and protects thetacky surface of the photohardenable layer.

[0054] As illustrated by FIG. 1(c), image-wise exposing thephotohardenable layer with actinic radiation through a patternedphotomask (106) causes detackification of the exposed areas of thephotohardenable layer (107) forming a pattern, for example a circuitpattern which would have electrically functional properties. The circuitpattern is a positive image wherein it would be the same as that foundon the photomask. After exposure, if present, the cover sheet on thephotohardenable layer is removed. FIG. 1(d) illustrates the transfersheet (thick film material side facing the imaged photohardenable layer)laminated onto the photohardenable layer (104) and (107). The thick filmcomposition (101) will substantially adhere to the unexposed tacky areasof the photohardenable layer. After peeling the used transfer sheet,which has a reverse circuit pattern formed thereon, off of thephotohardenable layer, a thick film circuit pattern is produced formingan article as illustrated in FIG. 1(e). The above process may berepeated, i.e., photohardenable layer, imaging, applying transfer sheet,at least once until desired layer number is reached. The article willthen undergo a firing step.

[0055] Optionally, depending on the application of the assembly, theassembly may undergo a heat treatment which causes the thick filmcircuit pattern to diffuse through the tacky non-hardenedphotohardenable layer onto the substrate surface. This is then followedby a firing step.

[0056] The presently available materials that make up thephotohardenable layer will be fired or burned-out at about 400° C. Thus,if complete burnout and removal of the photohardenable layer is desired,then the recommended firing temperature should be above 400° C.

[0057] In order to achieve adhesion of the thick film composition whenfiring in the 400 to 1000° C. temperature range, it was mentioned that aglass frit/inorganic binder system in the thick film composition isimportant. In some special cases, this requirement might not benecessary. When an inorganic binderless thick film composition isapplied to a substrate that contains dielectric or glassy componentswith a softening point close to the firing/sintering temperature of thebinderless thick film composition, then the substrate surface itself canreplace the role of the glass/inorganic powders in the traditional thickfilm composition. Additionally, if the conductive, resistive ordielectric powder itself is coated with some kind of glass or ceramic(or mixture thereof), this coating could act as the inorganic bindersystem of the thick film composition. The glass/ceramic coating can beapplied in a number of ways including spraying, solution dipping,aerosol reduction, precipitation, vapor deposition, tumbling and so on.The coated particles could be heat treated for a uniform and robustcoating.

[0058] In another embodiment, it is possible to skip the exposure orimaging step of the photohardable layer disposed on a substrate asdescribed above. In absence of an imaging step, once the cover sheet ofthe photohardenable layer is removed the entire surface of thephotohardenable layer will remain tacky. Upon lamination of a transfersheet to the tacky photohardenable layer and removal of the sheet, thethick film composition of the transfer sheet will substantially remainon the photohardenable layer. Therefore, the pattern created will befull coverage of the unexposed area. This is especially useful indielectric thick film composition applications.

[0059]FIG. 2, describes a polymer thick film composition patterningmethod using a negative image process. FIG. 2(a) depicts the polymerthick film composition (201) applied and dried on a suitable support(202) creating a transfer sheet. The support could be rigid or flexible,organic or inorganic based. This embodiment is particularly useful for aheat sensitive support, such as a printed wiring board. Although,supports that may withstand firing conditions can be used. The polymerthick film composition is applied on the support by any suitable means,including printing, casting, and spraying and then dried. In FIG. 2(b) aphotohardenable layer (which comes sandwiched between a base layer (205)and an optional cover sheet (204)), such as CROMALIN®, is image-wiseexposed through the cover sheet with a negative image of a desiredpattern. The cover sheet (204) is then removed. FIG. 2(c) depicts thelamination of the imaged photohardenable layer (203) onto the polymerthick film composition transfer sheet where the thick film compositionadheres to the support in the non tacky areas of the imaged layer andsubstantially adheres to the imaged layer in the tacky areas. In FIG.2(d) the imaged photohardenable layer and its base layer are then peeledoff, leaving behind a patterned polymer thick film composition (201) onthe support (202) forming a patterned article. The patterned polymerthick film composition is a negative image of the exposed photo-toolpattern. The patterned polymer thick film composition is then cured atthe desired time, temperature, atmosphere and pressure to achieve theproperties needed for the electronic application. Polymer thick filmcompositions useful in this application are well known and describedhereinabove.

[0060] The new processes described herein offer many advantages,including capabilities for large area substrates, precision and highdensity patterning, uniform metallization thickness across the wholesubstrate surface, automated mass production capabilities, applicable tovarious shape, type, flexible and rigid substrates (for example:polyester, polyolefin, polycarbonate, PVC, MYLAR®), TRESPAPHAN®,polystyrene, printed wiring board, laminates, BT, polyimide, paper,metal or other sheets, glass, ceramics-oxide and nonoxide, green-unfiredceramics and glass ceramics), sequential or singlefiring/cofiring/curing multilayer patterning and so on.

[0061] The invention, also, extends to a recovery process wherein a usedtransfer sheet that has undergone transfer processing resulting in areverse image of the desired circuit pattern is stripped of anyremaining thick film composition wherein the stripped thick filmcomposition could then be reclaimed. FIG. 3 illustrates an embodiment ofa recovery system that is a roll to roll system wherein the transfersheet is a continuous roll. A used transfer roll (301) is threadedthrough a double roller system (302) which allows for controlleddelivery of the transfer roll into a vat (303) which contains a solventbath that will dissolve the organics found in the thick film compositionwherein the thick film composition and the solvent form a solution. Forease of processing in a manufacturing facility, it is desirable to usethe same solvents that were used in making the thick film compositionfound on the transfer roll, but any compatible solvent that willdissolve the thick film composition may be used. An optional agitatingsource (304) may be used in the solvent vat. The agitating source may bean ultrasonic, agitator, scrubber or any other device that would agitatethe particles of the thick film composition for removable from thesupport. The uncoated support is then removed from the vat that containsparticles from the thick film composition solubilized by the solvent.The solution is drained from the vat and the rheology of the solution isadjusted which renders the solution suitable for casting on a supportfor the formation of new transfer sheets or transfer rolls.

EXAMPLES Formation of Transfer Sheet

[0062] Process for casting a thick film silver containing composition onTRESPAPHAN® support is described. The transfer sheet produced is used inExamples 1-4 and 8 hereinbelow unless otherwise specified in theexample. All percentages are in weight percent unless otherwise stated.

[0063] In a stoneware ceramic jar, the following ingredients were added:

[0064] Alumina Beads, filling the jar about 40 percent

[0065] 58.5 wt. % Organic Medium Composition

[0066] 37.5 wt. % Silver Powder (spherical silver, 0.1 to 3 micrometers)

[0067] 1.0 wt. % Frit Composition

[0068] 3.0 wt. % Ethyl acetate Organic Medium composition: Ethyl Acetate  82% Methyl Ethyl Ketone   6% Diethylene Glycol Dibutyl Ether   2%Dibutyl Phthalate  0.5% Ethyl Cellulose   2% VARCUM ® Resin*  7.5% FritComposition Silicon Dioxide   8% Aluminum Oxide  0.5% Lead Oxide   70%Boron Oxide 12.5% Zinc Oxide  6.5% Lead Fluoride  2.5%

[0069] The mixture is jar milled for 12 to 15 hours, the beads screenedand the composition is cast on a TRESPAPHAN® sheet made by HoechstTRESPAPHAN® of Winston-Salem, N.C., using a doctor blade with an openingof 15 micrometers.

[0070] The cast sheet is air dried for 15 minutes followed by ovendrying at 80° C. for 10 minutes. The silver-coated transfer sheet isready for use.

[0071] VARCUM® Resin available from Schenectady International,Schenectady, N.Y.

Example 1

[0072] A positive CROMALIN® film, type ICFD (made by E. I. du Pont deNemours and Company, Wilmington, Del.), having a coating weight in therange of 300 mg/dm², was laminated (after peeling the removable baselayer) to a soda lime glass substrate at about 250 d/F. The CROMALIN®film was exposed using a photomask circuit pattern (5KW UV bulb) for 15units. The cover sheet on the imaged CROMALIN® film was peeled off. Atransfer sheet was laminated onto the imaged film (with the thick filmside of the transfer sheet facing the imaged layer). Peeling thetransfer sheet produced the desired pattern where the unexposed areas ofthe imaged film layer remained tacky and the thick film was disposed onthe tacky areas. The structure was fired at 500° C. in air using astandard thick film firing profile for displays. The imaged film wascompletely burned out and the sintered, patterned thick film adheredstrongly to the soda lime glass substrate. There was no detectableadhesion difference between the above patterning approach and comparablywith the standard approach where the same silver thick film compositionis screen-printed and fired on the same type of glass substrate.

Example 2

[0073] The same process as Example 1 was repeated with the exception ofno exposure of a CROMALIN® film and processed at a firing temperature of585° C. The CROMALIN® film (completely covered by the thick filmcomposition of the transfer sheet) was burned out and adhesion of thethick film composition across the substrate surface was obtained.

Example 3

[0074] The same process as Example 2 was repeated 6 times (imagedfilm/thick film composition/imaged film/thick film composition, etc.)and the six-layer imaged film/thick film composition was cofired at 585°C. Similarly, all six layers of the imaged film were burned out and athick film layer adhered to the glass substrate.

Example 4

[0075] A double transfer sheet structure was produced. A first transfersheet as described in the transfer sheet section above was made,followed by casting and drying a black contrasting resistor composition,product DC243 sold by E. I. du Pont de Nemours and Company onto thethick film side of the first transfer sheet. The double layer transfersheet was processed according to Example 1 and fired in air at 585° C.

Example 5

[0076] A gold based thick film composition product QG150 sold by E. I.du Pont de Nemours and Company was thinned with 10% TEXANOL® sold byEastman Chemicals, Kingsport, Tenn. and cast on MYLAR® film to make atransfer sheet. The transfer sheet was processed according to Example 1on a 99.5 percent polished alumina substrate. The structure was fired inair at 850° C. The measured adhesion was good.

Example 6

[0077] A gold based thick film composition QG150 thinned as in Example 5and cast and dried onto TRESPAPHAN® film and processed according toExample 1 on a 96 percent alumina substrate. The resulting structure wasfired in air at 850° C. The measured adhesion was similar to a standardprocess wherein a thick film composition (QG150) is printed and fired onthe same type of substrate. Two circular patterns 1875 microns apartwere noted. The distance between the two circular patterns were measuredin the green state and after firing. The distance between the centers ofthe fired structure was only 3 microns different when compared to thegreen unfired structure.

Example 7

[0078] Thick film dielectric composition 5704 available from E. I. duPont de Nemours and Company was cast and dried onto a MYLAR® polyesterfilm support. A positive CROMALIN® sheet was then laminated onto analumina substrate. The CROMALIN® sheet was exposed to actinic radiationusing a photomask to create an imaged layer. The cover sheet on theCROMALIN® sheet was then peeled off exposing the imaged layer. The casttransfer sheet was laminated onto the imaged layer. Peeling the transfersheet produced the desired pattern on the imaged layer forming astructure. The desired pattern included separated lands and holes forpossible via applications. The structure was then fired at 850° C. inair.

Example 8

[0079] In order to facilitate recovery and re-use of expensive preciousmetal or phosphor powders, a thick film composition formulation as foundin the transfer sheet section hereinabove was divided and cast onto twoseparate TRESPAPHAN® sheets wherein a double-step application was used.Inorganic powders absent precious metals was dispersed in an organicmedium and cast onto a first TRESPAPHAN® carrier sheet. Then the silverprecious metal powders were dispersed in organic medium and cast onto asecond TRESPAPHAN® carrier sheet. Patterning was accomplished in twosteps. First, the inorganic coated TRESPAPHAN® sheet was laminated ontop of an imaged CROMALIN® sheet similar to the process described inExample 1. Second, the process was repeated using the silver metalcoated TRESPAPHAN® sheet laminated to a second imaged CROMALIN® sheetexposed with the same pattern as the first. The structure was then firedin air at 500° C. The resistivity of the fired silver pattern was foundto be similar when using either a single step transfer sheet application(for example, as found in Example 1) or double step transfer sheetapplication as given in this example.

Example 9

[0080] A multilayer capacitor build was accomplished by first casting awet stack of X7R type Barium Titanate based dielectric made by FerroCorporation, Riverside, Calif., onto a first MYLAR® polyester filmforming a dielectric sheet. A palladium/silver (27/73) conductoravailable from E. I. du Pont de Nemours and Company was cast onto asecond MYLAR® film forming a transfer sheet. A three conductor layercapacitor structure was built by first laminating two layers of thedielectric MYLAR® sheet, followed by lamination of a CROMALIN® sheetwhich was then imaged through a photomask; cover sheet was removed; thenthe palladium-silver conductor transfer sheet was laminated onto theimaged layer and peeled off leaving a patterned electrode layer. Thedielectric sheet is then laminated on top of the patterned layer. (Theneed for a tacky photohardenable layer (CROMALIN® sheet) between thepatterned electrode and the dielectric was not required; but may beadded based on the green strength characteristics of the dielectriclayer.) The process of CROMALIN® sheet/image/peel cover/laminateelectrode transfer sheet/peel was repeated three times to obtain a threelayer capacitor structure. The top layer is then topped by laminationwith two layers of dielectric. The whole structure is then removed fromthe MYLAR® base, fired in an oven at 1200° C., and cut into pieces. Theabove represents an alternate method of building multilayer (tens orhundreds of layers) capacitor structures using the desired buildingbase, suitable dielectric compositions and electrode conductorcompositions.

Example 10

[0081] A polymer thick film silver conductor composition (Product 5000available from E. I. du Pont de Nemours and Company) thinned withThinner 8260 (available from E. 1. du Pont de Nemours and Company) wasscreen printed onto a MYLAR® polyester film, such that the film iscovered with the conductor composition forming a transfer sheet. Theprinted thickness of the composition on the film was about 5 microns.The composition was dried at 50° C. for 10 minutes to drive off volatilesolvents. Separately, a CROMALIN® photohardenable layer (which comessandwiched between a support layer and a cover sheet) was imaged with anegative patterned image of a circuit pattern creating tacky andnon-tacky areas. The cover sheet is then removed and the imaged layer onthe support layer was laminated onto the composition side of thetransfer sheet. The imaged layer and its support layer and compositionthat stuck to the tacky areas of the imaged layer were peeled off,leaving behind a patterned polymer thick film composition on thesupport. The patterned polymer thick film was a negative image of thephoto-tool pattern. The patterned polymer thick film composition is thencured at 120° C. for 10 minutes to obtain the required adhesion andresistivity properties of the polymer thick film composition.

What is claimed is:
 1. A process for forming a pattern havingelectrically functional properties on a substrate comprising the stepsof: (a) providing a photosensitive layer having a tacky surface disposedon a substrate; (b) image-wise exposing the photosensitive tacky surfaceto form an imaged layer having tacky and non-tacky areas; (c) applying asheet comprising at least one layer of a thick film composition disposedon a support to the imaged layer wherein the imaged layer is in contactwith the thick film composition of the sheet; (d) removing the supportwherein the thick film composition remains on the support in thenon-tacky areas of the imaged layer and the thick film compositionsubstantially adheres to the tacky areas of the imaged layer forming apatterned article; and (e) firing the thick film composition of thepatterned article.
 2. A process for forming a pattern havingelectrically functional properties on a substrate comprising the stepsof: (a) providing a photosensitive layer having a tacky surface disposedon a substrate; (b) image-wise exposing the photosensitive tacky surfaceto form an imaged layer having tacky and non-tacky areas; (c) applyingthe exposed photosensitive layer to a sheet comprising at least onelayer of a thick film composition disposed on a support wherein theimaged layer is in contact with the thick film composition of the sheet;(d) removing the imaged layer wherein the thick film compositionsubstantially adheres to the tacky areas of the imaged layer and remainson the support in the non-tacky areas of the imaged layer forming apatterned article; (e) curing the thick film composition of thepatterned article.
 3. A process for forming a pattern havingelectrically functional properties on a substrate comprising the stepsof: (a) providing a layer having a tacky surface disposed on asubstrate; (b) applying a sheet comprising at least one layer of a thickfilm composition disposed on a support to the tacky surface wherein thetacky surface is in contact with the thick film composition of thesheet; (c) removing the support wherein the thick film compositionsubstantially adheres to the tacky surface forming a patterned article;and (d) firing the thick film composition of the patterned article. 4.The process of claims 1 or 2 wherein steps (a) through (d) are repeatedat least once.
 5. The process of claim 1 or 3 wherein the thick filmcomposition is a fireable thick film composition.
 6. The process ofclaim 2 wherein the thick film composition is a polymer thick filmcomposition.
 7. The process of claims 1, 2 or 3 wherein the thick filmcomposition comprises an oxalic acid catalyzed alkyl t-butyl/amylphenolic resin.
 8. The process of claims 1 or 2 wherein the substrate isselected from glass, glass-ceramic, low temperature co-fired ceramic,ceramic and coated substrates.
 9. The process of claims 1, 2 or 3wherein the photosensitive layer is photohardenable.
 10. The process ofclaims 1 or 3 wherein the firing step comprises a firing temperature ofat least 400° C.
 11. A process for the recovery of a thick filmcomposition from a support comprising a support having a dried thickfilm composition in the form of a reverse circuit pattern comprising thesequential steps of: (a) passing the dried thick film composition on afirst support through a solvent bath to form a solution of the thickfilm composition; (b) adjusting the viscosity of the solution to form acastable solution.; and (c) applying the castable solution on a secondsupport.
 12. The process of claim 11 further comprising a step whereinthe solution is periodically removed.
 13. The process of claim 11further comprising drying the solution on the second support wherein thesolvent is volatilized and the dried solution and the support form asheet.
 14. The process of claim 13 wherein the sheet is utilized in theprocess of claims 1, 2 or
 3. 15. An intermediate having a pattern forforming electrically functional properties comprising an imaged layerhaving tacky and non-tacky areas disposed on a substrate which areas arein contact with a thick film composition disposed on a support.
 16. Theintermediate of claim 15 wherein the thick film composition comprises afireable thick film composition.
 17. The intermediate of claim 15wherein the thick film composition comprises an adhesion promoter whichallows the composition to have greater adhesion to the tacky surfacethan to the support.
 18. The intermediate of claim 17 wherein theadhesion promoter is oxalic acid catalyzed alkyl t-butyl/amyl phenolicresin.
 19. The intermediate of claim 15 wherein the imaged layercomprises a photosensitive composition.
 20. The intermediate of claim 15wherein the thick film composition is a polymer thick film composition.21. A method for using a photosensitive layer to form a pattern havingelectrically functional properties with a thick film composition,comprising the steps of: (a) forming tacky and non-tacky areas on thesurface of a photosensitive layer by image-wise exposing thephotosensitive layer to actinic radiation to form an image-wise exposedsurface; (b) contacting the image-wise exposed surface with a thick filmcomposition which substantially adheres to the tacky areas but does notadhere to the nontacky areas to form a pattern of thick film compositionon the image-wise exposed surface; and (c) firing the thick filmcomposition on the image-wise exposed surface.
 22. The method of claim21 wherein the thick film composition comprises an adhesion promoterwhich allows the composition to have greater adhesion to the tacky areasthan to the support.
 23. The method of claim 21 wherein the adhesionpromoter is an oxalic acid catalyzed alkyl t-butyl/amyl phenolic resin.24. The method of claim 21 wherein the photosensitive layer isphotohardenable.
 25. A system for forming a circuit pattern comprising:(a) a photosensitive layer; and (b) a sheet having a thick filmcomposition which imparts electrically functional properties appliedthereto.
 26. The system of claim 25 wherein the thick film compositionis a fireable thick film composition.
 27. The system of claim 25 whereinthe thick film composition comprises an adhesion promoter which allowsthe composition to have greater adhesion to the tacky surface than tothe support.
 28. The system of claim 25 wherein the adhesion promoter isan oxalic acid catalyzed alkyl t-butyl/amyl phenolic resin.
 29. Thesystem of claim 25 wherein the photosensitive layer is photohardenable.